Understanding mechanisms that regulate memory in T cells is crucial for developing strategies to protect against epidemic and pandemic influenza viruses. The goal of this project is to determine the contribution of the selectin family of adhesion molecules (E, P, and L) and the selectin ligand, PSGL-1, to T cell memory Recently, we found that glycosylated, biologically active ligands for selectins are acquired by a subset of CD4 and CD8 effectors in the draining lymph nodes after influenza virus infection and represent the majority of responding T cells in the lungs including IFN-g or IL-17 producers, in vivo correlates of Th1 and Th17 cells, respectively. Moreover, we find that a consistent fraction of memory phenotype T cells express selectin-binding activity, suggesting that this is a stable heritable trait that distinguishes a subset of memory cells. In the absence of functional selectin ligands or PSGL-1, CD4 cell expansion and localization in the lungs after influenza virus infection is unaltered. However, CD4 effectors produce elevated levels of IFN-g and IL-17 suggesting a role for selectin ligands in dampening the effector response. Importantly, the development of memory CD4 cells with the capacity for secondary expansion after infection is impaired, possibly because of altered homeostatic regulation. Our data support the hypothesis that mechanisms regulated by selectins can be essential for the delivery of signals that control CD4 effector cell responses as well as the homeostatic maintenance of a subset of memory cells. We will investigate the novel function(s) of selectins in the regulation of effector and memory T cells in following specific Aims: 1) to determine if differences in selectin-binding capacity identify memory T cell subsets with distinct functional properties and homeostatic regulation;2) to investigate selectin-dependent responses of T cells and the role of selectinbinding capacity in the generation of memory cells after influenza virus infection;and 3) to identify mechanisms by which selectins regulate T cell homeostasis. We will take advantage of mice that are deficient in PSGL-1-/-, mice that lack PSGL-1 signaling, and mice that lack functional selectin ligands due to deficiency of the IV/VII fucosyl transferases. We will use IFN-g and IL-17 reporter mice, selectin ligand fusion proteins, and WT and engineered influenza viruses to enable us to assess the responses of polyclonal and TCR transgenic CD4 cells and make comparisons to CDS cells. In each of these Aims we will collaborate with Projects 1 and 2 to examine defined subsets of CD4 and CD8 cells in the influenza model. We will collaborate with project 4, which will determine the relevance of selectins in the tuberculosis model.